These days, imaging systems from medical engineering play an important role in the examination of patients. The representations of the inner organs and structures of the patient generated by the imaging systems are used for diagnosing causes of disease, for planning operations, when carrying out operations or else for preparing therapeutic measures. Examples for such imaging systems include ultrasound systems, x-ray computed tomography (CT) systems, positron emission tomography (PET) systems, single photon emission computed tomography (SPECT) systems, or magnetic resonance (MR) systems.
In the case of the last-mentioned MR systems, so-called local coils are usually used within the scope of an MR examination for receiving MR signals from the examination object. These local coils are MR reception antenna assemblies containing MR antenna elements, usually in the form of conductor loops. During the examination, these local coils are arranged relatively close to the body surface, as directly as possible on the organ or body part of the patient to be examined. In contrast to relatively large antennas that are arranged further away from the patient and generally used to generate an overall slice image through a patient, local coils are arranged closer to the regions of interest. As a result, the noise portion caused by electrical losses within the body of the patient is reduced, leading to the signal-to-noise ratio of a local coil being, as a matter of principle, better than that from a more distant antenna.
The MR signals received by the MR antenna elements are generally still pre-amplified in the local coil, conducted out of the central region of the MR installation by cables and fed to a receiver of an MR system or an MR signal processing apparatus. In the latter, the received data are then digitized and processed further for imaging.
The MR antenna elements present in the local coil are generally coupled symmetrically by the cables. The cables are often embodied as coaxial cables. However, symmetrical coupling is not always possible and, as a result, so-called sheath waves are created on the connected coaxial cables. These sheath waves lead to falsification of the measurement data and thus adversely affect the quality of the MR imaging. Moreover, these sheath waves can also lead to relatively strong heating of the coaxial cables. The heating may, on occasion, cause burns to the skin in the case of direct skin contact of the patient with the cable.
Sheath wave chokes are used on the coaxial cables in order to avoid sheath waves. In the design of such a sheath wave choke, use is made either of a so-called balun, i.e. a lambda/4 resonator, or a parallel resonant circuit. Part of the coaxial cable is wound up for the parallel resonant circuit. This winding is bridged with a capacitor and tuned to the MR imaging frequency. However, a disadvantage arising here is that a very long coaxial cable has to be used. Due to low frequency of the MR imaging, the baluns, as a matter of principle, have very large dimensions. Accordingly, a patient may experience discomfort due to the additional narrowing of the examination space as a result of the sheath wave chokes or due to the additional weight lying on the patient as a result of the sheath wave chokes. This applies, in particular, if a plurality of local coils or antenna elements are used for receiving MR signals. Moreover, the examination space within an MR installation is restricted, restricting the use of a multiplicity of coaxial cables with sheath wave chokes. The restriction on space is a particular consideration if the patient is moved on an associated couch apparatus. Furthermore, the sheath wave chokes result in increased costs.